1. Field of the Invention
The invention involves a photopolymerizable printing plate comprising a support, at least one photopolymerizable layer, an elastomeric layer containing at least one thermoplastic, elastomeric, block copolymer and at least one photoinitiator or a photoinitiator system, and a cover layer; and a flexographic printing form prepared from the plate.
2. Description of Related Art
Photopolymerizable printing plates are known for use in making flexographic printing forms. The printing surface is produced by exposing a photopolymerizable layer imagewise to actinic radiation and subsequently removing the unexposed, unphotopolymerized areas of the printing plate. Examples are found in the following patents: DE-C 22 15 090, U.S. Pat. No. 4,266,005, U.S. Pat. No. 4,320,188, U.S. Pat. No. 4,126,466 and U.S. Pat. No. 4,430,417. Such photopolymerizable printing plates usually comprise a support, an optional adhesive layer or other underlayer, one or more photopolymerizable layers, an optional, elastomeric, intermediate layer, and a cover layer.
A preferred method for making such multilayer photopolymerizable printing plates is a process in which a previously extruded photopolymerizable composition is fed into the nip of a calender and is calendered between a support and a cover element, thus forming a photopolymerizable layer between them. EP-B 0 084 851 discloses a preparation method for a multilayer photopolymerizable printing plate having an added elastomeric layer between the cover layer and the photopolymerizable layer.
The photopolymerizable layers contain polymeric binders, photopolymerizable monomers, photoinitiators, and added auxiliaries, such as plasticizers, fillers, stabilizers, etc. The polymeric binders are usually thermoplastic, elastomeric, block copolymers, as described, for example, in DE-C 22 15 090. These are generally A-B-A block copolymers having thermoplastic blocks A and elastomeric blocks B, particularly linear and radial block copolymers with polystyrene end blocks, such as, for example, polystyrene/polyisoprene/polystyrene, (polystyrene/polyisoprene).sub.4 Si, or the corresponding butadiene polymers. The use of polymers having a certain vinyl content is also known for special purposes, such as, for example, for improving printing plate properties with special monomers (EP 0 525 206) or for preparing printing plates without monomer addition. Similar block copolymers and their use in highway engineering and automobile manufacture are described in EP-B 0 362 850. However, polystyrene/polybutadiene/polystyrene and polystyrene/ polyisoprene/polystyrene block copolymers are preferred for making flexographic printing forms.
Nevertheless, current state-of-the-art printing forms frequently do not meet requirements for flexographic printing. In particular, when UV-hardenable inks are used, the printing forms are not sufficiently resistant to ink constituents diffusing into the forms. This increases layer thickness, changes image geometry on the printing form surface, and decreases Shore A hardness during printing. This is seen in the printed image as undesired spreading in positive image elements or as fine negative image elements running together.
Therefore, the present invention is based on the problem of making available flexographic printing forms that have improved resistance to UV-hardenable printing inks and that do not show the disadvantages described for printing forms of the current state-of-the-art, without adversely affecting other essential properties of the photopolymerizable printing plates or flexographic printing forms.